This invention relates to extracting a heavy oil from a field sample of the oil and/or oil-containing portion of a subterranean oil formation and preparing a substantially solids-free oil sample having a chemical composition which is substantially identical to that of the oil in the reservoir.
As far as applicants have been able to ascertain, the methods of separating such oils from field samples and preparing samples for laboratory utilizations have remained substantially the same for at lest about 40 years. For example, the textbook "Petroleum Production Engineering Oil Field Development" by Lester Charles Uren, McGraw-Hill Book Company, Inc., 1946, describes a procedure for extracting oil from field samples. It comprises contacting the sample in a Soxhlet extractor with substantially any volatile solvent which does not alter the mineral structure of the reservoir material and is capable of dissolving the oil or oil residue from the reservoir material. In a booklet, "Syncrude Analytical Methods for Oil Sand and Bitumen Processing", published by Syncrude Canada, Ltd., August, 1979, the extraction procedure is substantially the same--"The sample is separated into bitumen, water and solids by refluxing with toluene in a solids extraction apparatus. Condensed solid and co-distilled water are continuously separated in a trap, the water being retained in the graduated section" (page 46).
Such prior procedures are relatively widely used but have a serious defect. It is generally desirable to mechanically separate the laboratory sample of the oil from solid particles large than about 0.1 micron; for example, by filtration through a millipore filter or by means of centrifugation. Due to the high viscosity of heavy oils, their dilution with the volatile solvent is usually required. After separating the solid particles, the solvent is removed by evaporation. The evaporation removes most of the water which is present in the original oil and also removes most or all of the volatile components that were present in the oil. Thus, in such prior procedures, the light ends are irretrievably lost and the hydrocarbon distribution within the solids-free sample of the oil is different from that in the original oil. These differences are particularly important in tests of the mobility of the oil in cores or packs at different temperatures and/or in contact with different fluids.